In a phased array ultrasound imaging system, an ultrasound transducer includes an array of N transducer elements. The system includes N parallel channels, each having a transmitter and a receiver connected to one of the transducer array elements. Each transmitter outputs an ultrasound pulse through the transducer element into an object being imaged, typically the human body. The transmitted ultrasound energy is steered and focused by applying appropriate delays to the pulses transmitted from each array element so that the transmitted energy adds constructively at a desired point. The transmitted ultrasound energy is partially reflected back to the transducer array by various structures and tissues in the body.
Steering and focusing of the received ultrasound energy are effected in a reverse manner. The reflected ultrasound energy from an object or structure arrives at the array elements at different times. The received signals are amplified and delayed in separate processing channels and then combined in a receive beamformer. The delay for each channel is selected such that the receive beam is steered at a desired angle and focused at a desired point. The delays may be varied dynamically so as to focus the beam at progressively increasing depths, or ranges, as the ultrasound energy is received. The transmitted beam is scanned over a region of the body, and the signals generated by the beamformer are processed to produce an image of the region.
In order to effect focusing and steering of the receive beam, delays must be applied to the received signals in each processing channel. The required delays vary with the steering angle of the receive beam, the position of each transducer element in the array, and with focal depth. Dynamic focusing is effected by varying the delays with time during reception of ultrasound echoes from progressively increasing depths. A typical phased array ultrasound transducer may include 128 elements or more. Thus, the computation and control of the required delay for each transducer element to effect dynamic focusing at a desired steering angle is difficult.
In one prior art approach, disclosed in U.S. Pat. No. 4,949,259 issued Aug. 14, 1990 to Hunt et al, the region being imaged is divided into zones at different depths from the transducer, and a delay is associated with each transducer element in each zone. This approach reduces the required number of delay coefficients compared to an ideal continuously-adjusted delay. However, since each delay is exactly correct at only one depth in the zone, the image quality is somewhat degraded.
U.S. Pat. No. 4,173,007, issued Oct. 30, 1979 to McKeighen et al., discloses an ultrasound imaging system using a memory with separate read and write capabilities to produce a dynamically variable delay. The delay can be varied by modifying the write or the read address pointer.
U.S. Pat. No. 5,111,695, issued May 12, 1992 to Engeler et al, discloses a method for dynamic phase focus of received energy for coherent imaging beam formation. The channel time delay is adjusted by apparatus with means for counting range clock signals, responsive to the initial steering angle, and a logic means for issuing fine time delay adjustment signals responsive to a phase control algorithm. Because of an approximation, the delays are not determined exactly.
Other prior art techniques for dynamic focusing are disclosed in U.S. Pat. No. 4,974,211 issued Nov. 27, 1990 to Corl; U.S. Pat. No. 5,113,706 issued May 19, 1992 to Pittaro; U.S. Pat. No. 4,870,971 issued Oct. 3, 1989 to Russell et al.; U.S. Pat. No. 4,707,813 issued Nov. 17, 1987 to Moeller et al.; and U.S. Pat. No. 4,227,417 issued Oct. 14, 1980 to Glenn.